Based on appearance alone, it typically is difficult or impossible to determine whether a conventional door lock is in a locked or unlocked condition. Nevertheless, there can be a regular need to make this determination. For example, because locking doors is a matter of routine that often is performed with little thought or attention, it is relatively common for an individual exiting a door to walk away and then immediately forget whether he or she did or did not lock the door. To verify the status of a conventional door lock, the individual typically must retrieve the appropriate key, insert the key into the door lock, turn the key, and then withdraw the key. This can amount to significant wasted effort. As another example, when approaching a door to gain entry, an individual may wish to know whether he or she can proceed to open the door or whether it will be necessary to first unlock the door, particularly if others have had access to the door since the individual last exited the door. Without knowing whether the door is in a locked or unlocked condition, the individual may waste effort attempting to open the door while it is in a locked condition or attempting to unlock the door while it is in an unlocked condition.
The need to determine whether a door lock is in a locked or unlocked condition can be particularly significant when a door has multiple locks. An individual approaching such a door may find that it does not open even if one of its locks is in an unlocked condition. As a result, the individual may attempt to unlock a door lock that is already in an unlocked condition. This action can actually lock the previously unlocked door lock, resulting in frustration and additional wasted effort.
In view of the scenarios discussed above, there is a need for devices that facilitate determining the status of door locks. Some conventional devices have attempted to address this need. For example, U.S. Pat. No. 4,559,796 (US'796) discloses “a status indicator for a door lock.” US'796 abstract. The status indicator disclosed in US'796 is deficient, in part, because it cannot be installed on an existing door lock without first disassembling the door lock assembly. US'796, column 4, lines 41-48. As another example, U.S. Pat. No. 6,255,957 (US'957) discloses a system including a key with a display and sensors that interact with markers in a door lock. US'957, abstract. The system disclosed in US'957 is deficient, in part, due to its complexity, its expense, and its inability to account for operation of the door lock with more than one key.